includes only free dissolved oxygen, not that
which is bound in compounds

one of the most fundamental parameters of a
lake

>90% of aquatic organisms require it

is a vital player in biochemical reactions;
e.g., under oxic conditions, phosphates are tied up in
bottom muds; under anoxic conditions, phosphorous becomes
soluble and circulates, triggering algal blooms.

solubility-exhibits an inverse
nonlinear relationship: as temperature increases, the amount of
oxygen water can hold decreases.

·Hence, at 4°C, water is saturated (100%) with oxygen,
holding 10.92 mg/l.

oNota bene: 100% saturation does not mean that no
more O2 can be held in solution. I have measured DO >200%.
Does this mean that bubbles should be forming? Not necessarily.
Saturation here means that 10.92 mg/l can be held at equilibrium; if
200% is produced by intense photosynthetic activity, the extra amount
will be lost (diffused) at the air/water interface.

Oxygen Solubility in Water at 760 mm Hg
pressure

Temp. °C

Oxygen (mg/L)

Temp.°C

Oxygen (mg/L)

0

14.16

18

9.18

1

13.77

19

9.01

2

13.40

20

8.84

3

13.05

21

8.68

4

12.70

22

8.53

5

12.37

23

8.38

6

12.06

24

8.25

7

11.76

25

8.11

8

11.47

26

7.99

9

11.19

27

7.86

10

10.92

28

7.75

11

10.67

29

7.64

12

10.43

30

7.53

13

10.20

31

7.42

14

9.98

32

7.32

15

9.76

33

7.22

16

9.56

34

7.13

17

9.37

35

7.04

·
is effected by altitude (air pressure); at higher altitudes there
is less air pressure and less O2. Saturation is usually
figured for sea level pressure (760 mm = 1 ATM). What is our altitude?

·
pressure exerts a role, too; recall that pressure increases 1
ATM/10 m; at depth there is much more
pressure on gases to stay in solution. Hence, it is hard for bubbles to
form against it. Therefore it takes very high supersaturation rates
(compared to surface pressure) to cause bubble formation. (Think of a
sealed vs. open bottle of beer.)

a nomogram can be used to
determine degree of saturation; use a straightedge to connect the
water temperature and DO. Read the % saturation at the intersection
of this line with the middle line.

Dissolved
Oxygen % Saturation Nomogram

·
at 10 meters, with a temperature of 10°C, at
surface pressure would hold (at 100% saturation) 10.92 mg, but you may
find 15 mg/l.; compared to the surface it would be supersaturated,
but at the depth and pressure it’s at, it may be less than saturated.

oHow can water be supersaturated?

§intense photosynthesis

§entrainment of air falling over a dam or spillway; high
pressure of impact drives gases into solution; may lead to gas bubble
disease, a problem in TVA dams

§affects fish if subjected for a few hours to >115%
saturation; bubbles form in tissues; emboli collect in gills causing
anoxia and death; also affects cladocerans. Other biota, e.g., crayfish
and stoneflies are hardier.

·
effect of salinity.

o
oceanic salinity averages ~3.5% (35 ‰);

o
at the same temperature, saltwater holds about 20% less O2
when saturated; e.g., at 0°C freshwater is saturated at
14.2 mg/l, while saltwater is saturated at 12.0 mg/l.

Sources of gain/loss of O2 water

Gain

dissolving at air/water interface

a very slow process, would take years for
any to reach 5 m;

turbulence carries O2 from
surface to depth; how far depends on how much of the lake is
circulating; wind-driven waves and currents are critical.

Note: agitation and turbulence are a
source of loss of O2 if waters are supersaturated.

photosynthesis: 6 CO2 +
6 H2O ® C6H12O6
+ 6O2

inflow of oxygen rich water

Loss

same as (1) above

respiration by plants, animals, microbes

6
O2 + C6H12O6® 6 CO2 + 6 H2O

3. inflow of O2 poor water that
may have a BOD/COD that uses up available O2.

4. increasing temperatures.

Oxygen Profiles

take oxygen levels at all depths (1 m
intervals) to develop the profile.

Clinograde profile. Dashed line represents temperature; solid line
is D.O

most O2 loss is at the
water/sediment interface

respiration use of O2 occursthroughout the water column; it is highest where most
organisms are, in the photic zone. There, however, O2
production is far in excess of use.

where does O2 production = O2
use? The compensation point where ~2% of sunlight
reaches (approximately the Secchi depth); therefore, at the
bottom of the photic zone.

Recall that O2 can be used in
non-biological, strictly chemical reactions, the chemical oxygen
demand (COD).

Deviations from these profiles:

Heterograde curves (hetero =
different)

caused by high/low concentration of DO at
seemingly unlikely places

e.g., positive heterograde, caused by a
metalimnetic O2 maximum-usually due to high
concentration of photosynthesizers at metalimnion. Why there?
Species adapted to low light and low temperature make use of more
nutrients in the metalimnion compared to the epilimnion, e.g., the
cyanophyte, Oscillatoria.

if there
is any DO in the water a second reaction between the Mn(OH)2
and DO occurs immediately to form a brownish manganic
oxide floc:

2Mn(OH)2
+ O2® 2MnO(OH)2(brown)

a further reaction with potassium
iodide releases an amount of iodine equivalent to the
original oxygen.

2Mn(SO4)
2 + 4KI ® 2MnSO4
+ 2K2SO4 + 2I2

if any bubbles are present up to this
point, the sample must be discarded and you must begin
again.

from this time on you are titrating the
iodine, so the sample can no longer be contaminated by
bubbles or atmospheric oxygen; hence, it can be worked up at
a later time.

using sodium thiosulfate, titrate the
solution from iodine color to clear; to aid in finding the
endpoint accurately, starch may be added. This will turn
the mixture blue-black; when the color disappears, the
endpoint has been reached.

high levels of nitrate, as well as
hydrogen sulfide and reducing materials will interfere,
yielding inaccurate results.

only as good as your reagents are. The
critical one is the titrant (PAO or sodium thiosulphate), therefore,
must standardize, i.e., compare your titrant to a sample of known
amount. Directions for this are in "Standard Methods", aka The
Bible.

many things can cause erroneous determinations

not your fault-interferences-azide removes
nitrates which are common in polluted water; other interferences
are iron and high suspended solids. May cause too much or
too little I2 to be released.